Warning : Use the following information at your own risk.  While accuracy is one my goals, there is always the possibility that some of the information could be wrong.  There could be typos.  I could also be severely mistaken in some of my knowledge. This site is meant to help clarify certain concepts of ECG and at no point should any life-or-death decision be made based upon the information contained within.  Remember, this is just some page on the internet.  (If you do find errors, please notify me by feedback.)

Atrial rhythms are rhythms that originate in the atria.  The atrial rhythms that fall under the category of PSVT are mentioned elsewhere. 

What is the difference between an electrical beat originating in the sinus and one originating in the atria?  For one, the P wave tends to be a different shape.  Sinus P waves tend to be rounded, upright, etc. while atria P waves tend to be weird shaped.  Sometimes they are pointy, sometime flat.  They can have a notch running down the middle of them.  Some are diphasic, which means that one part is above the isoelectric line while one part is below it. 

Premature atrial complex

When you feel your heart has "skipped a beat," it very well may have been due to a PAC.  A premature atrial complex (PAC) describes a wave or set of waves caused by an atrial pacemaker that interrupts the underlying rhythm.  Figure 6-1 shows a sinus rhythm that is twice interrupted by PACs.  The PACs shown consist of an atrial P wave along with a normal QRS complex and a normal T wave.  Compare the P waves on both the sinus complexes and the PACs. 

Premature describes the fact that the beat occurred before the regular one would have.  In figure 6-1, you can imagine that the sinus pacemaker was firing along at a regular rhythm until it was unexpectedly (and rudely) interrupted by the atrial pacemaker.  The atrial pacemaker causes the wave of depolarization that resets the sinus node.  Look at the R-R intervals.  The ones from the sinus to the following atrial complexes are shorter than the others.   

When an atrial pacemaker fire prematurely, it is entirely possible that the AV node is still refractory.  In these situations, you might find a single atrial P wave without the QRS complex and T waves.  We describe this as non-conducted. 

When these impulses conduct, you may see either a normal QRS or a wide QRS.  The wide QRS in this case is due to what is called aberrancy.  This is often due to one of the bundle branches still being refractory.

PACs are also covered in Section 10 : Premature Complexes.

Atrial fibrillation

Atrial fibrillation (often called "a-fib") is relatively common among the elderly.  It occurs when the electricity in the atria follows a seemingly random and repetitive path, causing the atria to quiver.  (The word fibrillation means quivering.)  Because the atria are quivering, they are unable to pump blood.  The ventricles are still functional, but are depolarized at irregular intervals.  This rhythm can last for years and is not always symptomatic. 

There are a few reasons why this rhythm is bad.  Some of them are :

One of the major questions that doctors are asking themselves is : is it better to control or convert? 

Control refers to controlling the rate.  Keeping the rate of ventricular contraction under 100/min. often minimizes the symptoms of this rhythm.  This is usually done with medication.  When the rate of QRS complexes (and thus ventricular depolarizations) exceeds 100, we call this rhythm uncontrolled atrial fibrillation.

Convert refers to converting the rhythm into a sinus rhythm.  You might think this would be the obvious choice.  The problem is, however, that someone who has been in atrial fibrillation for a while has a high chance of "throwing clots" in the left atrium.  These thrombi (clots) are thought to form in the atria when they are fibrillating.  In the process of conversion to a sinus rhythm, the thrombi may become dislodged from the atrium. They are then likely to be sent through the left ventricle, the aorta, and into the arteries that supply the brain.  This could lead to a stroke.   Patients who undergo this "cardioversion" are often put on anticoagulants for weeks before the procedure.    

There are usually two readily apparent things in an ECG of atrial fibrillation. 

1. There is an extremely irregular QRS rate.  The R-to-R values often differ with each beat with no visible pattern to their timing. 
2. There are no easily discernible P waves.  Instead, the baseline usually appears chaotic.  It may have an appearance ranging from coarse (large and jagged)  to fine (relatively smooth).   This is the atrial contribution to the ECG.     

The PRI cannot be measured. 

Atrial flutter

The buzzword that everyone loves to use when describing atrial flutter is "saw tooth."  The term is used for good reason : the P waves often looks like the teeth of a saw when viewed in Lead II.  These P waves are often called "flutter waves."

In atrial flutter, the atria are depolarizing at an extremely rapid rate.  The AV node will normally only conduct impulses up to a rate around 220 per minute.  In atrial flutter, the atria are depolarizing about 250 - 350 times per minute.  This means that not all of the impulses will be conducted.  The conduction ratio (P:QRS ratio) is often relatively constant at 2:1 or 4:1, although it can also vary.  With 2:1 conduction, the ventricular rate is usually about 150.  Unlike atrial fibrillation, atrial flutter QRS complexes tend to appear at regular intervals.

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